Peripheral definition - c

Question

Could someone please explain the following construction to a beginner:
typedef struct
{
__IO uint32_t CTRL; /**< Control Register */
__IO uint32_t CNT; /**< Counter Value Register */
__IO uint32_t COMP0; /**< Compare Value Register 0 */
__IO uint32_t COMP1; /**< Compare Value Register 1 */
__I uint32_t IF; /**< Interrupt Flag Register */
__IO uint32_t IFS; /**< Interrupt Flag Set Register */
__IO uint32_t IFC; /**< Interrupt Flag Clear Register */
__IO uint32_t IEN; /**< Interrupt Enable Register */
__IO uint32_t FREEZE; /**< Freeze Register */
__I uint32_t SYNCBUSY; /**< Synchronization Busy Register */
} RTC_TypeDef;
#define RTC_BASE (0x40080000UL)
#define RTC ((RTC_TypeDef *) RTC_BASE)
especially the last line
Why are the brackets so unusual? What does the * mean? Pointer or multiply operator ?
Thanks

Answer 1

Why are the brackets so unusual? What does the * mean? Pointer or
multiply operator ?
* here is a pointer not multiplication operator.
There is nothing unusual here about the macro. It is just type casting the address to be of type struct RTC_TypeDef

Answer 2

Whenever you will encounter a single or multiple * after a datatype (even a user defined datatype), its a pointer.
What you are calling a constructions:
typedef struct
{
__IO uint32_t CTRL; /**< Control Register */
__IO uint32_t CNT; /**< Counter Value Register */
__IO uint32_t COMP0; /**< Compare Value Register 0 */
__IO uint32_t COMP1; /**< Compare Value Register 1 */
__I uint32_t IF; /**< Interrupt Flag Register */
__IO uint32_t IFS; /**< Interrupt Flag Set Register */
__IO uint32_t IFC; /**< Interrupt Flag Clear Register */
__IO uint32_t IEN; /**< Interrupt Enable Register */
__IO uint32_t FREEZE; /**< Freeze Register */
__I uint32_t SYNCBUSY; /**< Synchronization Busy Register */
} RTC_TypeDef;
is a structure and programmatically called a struct, a user-defined datatype. struct is keyword in C. And so is typedef. To keep it simple as of now, I'll say, typedef here is specifically used to have the convenience to write RTC_TypeDef; instead of having the need to write struct stRTC for example.
RTC_BASE is a macro. So, before compilation pre-processor will replace RTC_BASE with the value (0x40080000UL), again, it is for convenience and readability.
This last line:
#define RTC ((RTC_TypeDef *) RTC_BASE)
also is a macro defined. Which is nothing but a pointer of the RTC_Typedef type, pointing to whatever is located at the address (0x40080000UL)
So, before the compilation the pre-processor will replace all instances of RTC with ((RTC_TypeDef *)0x40080000UL).
If you increment RTC pointer, it will increment by sizeof(RTC_Typedef).

Answer 3

The structure definition is pretty straight forward, variables are encapsulated in a structure. That structure can then be referenced as RTC_TypeDef because you use typedef.
typedef struct
{
__IO uint32_t CTRL; /**< Control Register */
__IO uint32_t CNT; /**< Counter Value Register */
__IO uint32_t COMP0; /**< Compare Value Register 0 */
__IO uint32_t COMP1; /**< Compare Value Register 1 */
__I uint32_t IF; /**< Interrupt Flag Register */
__IO uint32_t IFS; /**< Interrupt Flag Set Register */
__IO uint32_t IFC; /**< Interrupt Flag Clear Register */
__IO uint32_t IEN; /**< Interrupt Enable Register */
__IO uint32_t FREEZE; /**< Freeze Register */
__I uint32_t SYNCBUSY; /**< Synchronization Busy Register */
} RTC_TypeDef; /** #} */
The following line means every time you write RTC_BASE, the compiler will replace that line with (0x40080000UL)
#define RTC_BASE (0x40080000UL)
And finally, the following line can be broken into two parts:
(RTB_TypeDef *)RTC_BASE
Which means (RTB_TypeDef *)(0x40080000UL) - basically tells the compiler to treat memory address 0x40080000UL as a pointer to RTB_TypeDef
and the extra parenthesis are there to denote a single variable as a compund expression RTC.
From that point on, whenever the compiler sees RTC, it will replace it with (RTB_TypeDef *)(0x40080000UL), basically a convenient way to mark the beginning of this structure in memory